1. Field of the Invention
This invention relates to a lead-free soldering composition, in particular, a lead-free soldering composition suitable for soldering components in electronic and microelectronic circuitry.
2. Description of Prior Art
Soldering compositions or alloys are low-melting-temperature conducting alloys which join two (2) conductors by wetting their surfaces and then solidifying to a mechanically strong solid. Solders loose their mechanical strength above a solidus temperature and become sufficiently fluid at about 50.degree. C. above a liquidus temperature. Solders suitable for use in electronic applications should he strong and creep and corrosion-resistant and should he reasonable electrical conductors. Due to the characteristics thereof, lead-tin solders are most commonly used for electronic applications. The eutectie composition (63% Sn-37% Pb) has good mechanical properties combined with high wetability. We are unaware of any lead-free solders having characteristics similar to lead-tin solders suitable for use in electronic applications. It is, indeed, unexpected and unanticipated that the solder of this invention, being a lead-free solder, is suitable for use in electronic applications.
It is particularly surprising because the lead-free solder of this invention requires no additional components, such as viscosity controlling agents. For example, the "Guide to INDALLOY Specialty Solders" lists 223 solder compositions, only three (3) of which operate within the temperature range of 188.degree. C. to 197.degree. C., all of which contain lead.
It will also be apparent to those skilled in the art that the interaction between the elements of an alloy, particularly with respect to producing an alloy having the properties required for soldering electronic components, is not predictable, nor can such interaction be determined on the basis of known existing empirical data. That is, the properties of an alloy produced by mixing two or more known elements cannot be predicted based upon knowledge of the properties of the individual elements.
U.S. Pat. No. 4,373,974 teaches a solder composition or a solder cream for soldering components in electronic circuitry in which finely divided solder metals, such as tin and lead, are disbursed in a vehicle containing viscosity controlling agents, organic solvents, and a rosin or rosin derivative as a flux. The vehicle for the solder metals of the solder composition is a liquid composition having desirable viscosity characteristics and which maintains the solder metals in suspension without settling of the fine solder metal particles. U.S. Pat. No. 4,919,729 teaches a solder paste for use in a reducing atmosphere, the solder including a polyhydric alcoholic binder which vaporizes or decomposes before the solder liquifies during heating and a solder powder containing at least two (2) of the elements tin, bismuth, cadmium, antimony, indium, lead, silver, and gold. The polyhydric alcoholic binder is required to reduce solder splattering during heating. The solder paste is reflowed in a reducing atmosphere which serves as a flux.
U.S. Pat. No. 5,120,498 teaches a solder composition in which heavy metal solder compositions consisting of alloys from within the heavy metal system Pb-Sn-In-Bi-Cd-Sb-Hg-Ga-Ag-Au are combined with a small amount of a light reactive metal selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, calcium, and magnesium. The solders in accordance with this patent have exceptional ability to wet non-metals, in particular glass.
U.S. Pat. No. 4,929,423 teaches a solder containing a lead-free alloy for use as a plumbing solder comprising bismuth, copper, silver, phosphorus, tin and a rare earth mixture. Similarly, U.S. Pat. No. 4,758,407 teaches a lead-free, cadmium free, tin and tin/antimony based solder alloy for joining copper tubes and brass pipe and fittings. U.S. Pat. No. 4,980,243 teaches a silver alloy solder for direct bonding of ceramic parts. U.S. Pat. No. 4,966,142 teaches an alloy for joining superconductive materials to themselves, conductive materials, or semi-conductive materials, having 10 to 90 atomic percent indium combined with mercury, gallium, bismuth, cadmium, tin, or lead. U.S. Pat. No. 5,077,005 teaches a high-conductivity copper alloy for forming wires, the alloy containing copper and at least one of indium, cadmium, antimony, bismuth, titanium, silver, tin, lead, zirconium, and hafnium. Finally, U.S. Pat. No. 3,909,312 teaches a non-corrosive flux for relatively soft solders comprising a mixture of ammonia phosphates, ammonia fluoride, and a resin.
None of the prior art references of which we are aware teach or suggest a lead-free solder suitable for use in electronic applications and, in particular, suitable as a replacement for the most commonly used lead-tin solders, in particular a 63% Sn-37% Pb solder.